High speed serial (“HSS”) data communication is constantly increasing in importance. It permits more efficient pin usage and power consumption than traditional parallel data communication, while preserving or increasing available bandwidth.
The extremely fast data rate associated with HSS communication makes the verification of signal integrity an important concern. The receiving end of a link must know when the sending end has ceased data transmission, either intentionally or as a result of system failure or signal degradation. Thus, effective signal detection is necessary for reliable HSS communication.
One simple solution would be to introduce an extra signal that runs parallel to the transmitted data. This binary signal would indicate whether or not valid data was being sent at a given time. However, this solution wastes valuable interface pins at the edge of a chip, and does not address the situation where data is unintentionally corrupted.
If the data being transmitted is encoded in a conventional binary fashion, with logic 0 represented by ground (“GND”) and logic 1 represented by the power supply voltage (“VCC”), then another possibility would be to examine the incoming data for logic 1's. After a certain amount of a time has passed, during which no logic 1's have been received, the receiving circuit can safely conclude that data transmission has ceased. However, this solution will not work if any type of automatic gain control is used. In this case, even after data transmission has stopped, the automatic gain control would amplify the magnitude of the GND voltage being transmitted, so that the received data is nothing more than amplified white noise passing for valid data.
In order to accommodate the use of automatic gain control, one must turn to slightly more complex solutions. Many successful strategies exist, and are often used in combination with each other. For example, the data to be sent can be encoded in a way that will introduce more bits into the transmission stream. Because of the added redundancy, random errors in the data will tend to violate the coding rules. Thus, if the receiver detects many violations in succession, it can reasonably assume that transmission has ceased.
Another common solution, and the one most relevant to this invention, relies on voltage envelope detection. It extracts a slowly varying envelope based on the peak-to-peak amplitude of the received signal. If the voltage level of the envelope is less than a certain pre-determined threshold, the receiver concludes that the transmitter has gone down.
Existing signal detectors employing envelope detection are typically common-mode dependent, making them less robust. Different hardware vendors may use slightly different VCC voltages to indicate a logical high, so it is difficult to design circuitry that can accommodate all vendors. In addition, absolute voltages are difficult to set precisely, which makes tasks like fixing a reliable threshold voltage more difficult.
In view of the foregoing, it would be desirable to provide more robust circuitry and methods for signal detection that are independent of common-mode voltage.